1. Field of the Invention
The invention relates generally to optical communications systems, and more particularly to bidirectional transmission of optical signals.
2. Description of the Related Art
Various fiber optic communication systems have been developed for conveying information bidirectionally to and from a first location and a second location. Some of these systems employ a laser only at the first location, whereas, at the second location, an unused portion of the laser light is "ooped back" to the first location. This looped back laser light may be encoded with data by means of an optical modulator. Examples of such systems are disclosed in U.S. Pat. No. 4,879,763, entitled, "Optical Fiber Bidirectional Transmission System," and U.S. Pat. No. 5,559,624, entitled, "Communication System Based on Remote Interrogation of Terminal Equipment". One advantage of these optical communication systems is that a laser source is only required at one end of the system, while less expensive modulators can be used at the other end.
Bidirectional optical communication systems using looped back laser light are useful in applications such as wavelength-division-multiplexed (WDM) access networks. WDM networks employ a laser at a first location, which is typically a central office, and utilize a plurality of Optical Network Units (ONU's) situated at a plurality of second locations. The ONU's are commonly located at or near customer premises, where environmental conditions such as temperature and humidity may vary over a relatively wide range. For this reason, it is difficult to provide accurate wavelength registration at the ONU locations. The looped back laser light provides automatic wavelength registration at the ONU locations, because the same wavelength of light that was transmitted from the laser at the central office is sent back to the central office by the ONU.
One shortcoming of using looped back laser light to provide a bidirectional communications system is that coherent Rayleigh noise significantly limits the performance of the system. The phenomenon of Coherent Rayleigh noise is discussed in greater detail in U.S. Pat. No. 4,879,763, issued to Thomas H. Wood, and also in an article by T. H. Wood, R. A. Linke, B. L. Kaspar, and E. C. Carr, "Observation of coherent Rayleigh noise in single-source bidirectional optical fiber system," Journ. Lightwave Techn., Vol. 6, pp. 346-352 (1988). Coherent Rayleigh Noise (CRN) arises when light traveling from the central office laser, to the modulator, and back to a detector at the central office, interferes with light that was back scattered by the fiber optic cable throughout the entire length of the fiber. Although the scattered signal and the signal travelling to and from the central office (the looped back signal) both originate from the same source, these signals have each encountered different delays between the laser and the detector. Therefore, the scattered signal beats with the looped-back signal, thereby generating a noise spectrum that is roughly proportional to the laser line width. Beat interference is also caused by discrete reflections from optical fiber splices, optical couplers, and optical splitters that exist between the central office and an ONU.
Substantially all of the noise spectra within the electrical bandwidth of a detector will be detected by that detector. Since the detector bandwidth is tied to the communications bit rate, the minimum electrical bandwidth of the detector is usually fixed. For example, for NRZ (non return to zero) signals, the electrical bandwidth of the receiver cannot be made less than 0.7 times the bit rate. Therefore, beyond a certain point, one cannot improve signal-to-noise ratio merely by reducing the detector bandwidth. If one is already operating at the minimum practicable detector bandwidth as determined by the desired bit rate of a given system, one must resort to other techniques to improve signal-to-noise ratio.
Various techniques have been developed to address the problem of coherent Rayleigh noise in bidirectional optical communications systems. These techniques have been adapted for an operational environment where a continuous-wave laser, such as a Fabry-Perot or distributed-feedback (DFB) semiconductor laser, is used as the light source at the Central Office. One approach to minimizing coherent Rayleigh noise, involving the use of separate fibers for up- and downstream traffic, is disclosed in U.S. Pat. No. 5,559,624, entitled, "Communication System Based on Remote Interrogation of Terminal Equipment". Although effective at eliminating the adverse consequences of Rayleigh noise, this solution adds considerably to the cost of the system and negates the simplicity of using a laser only at one end of the communications system.
Another prior art solution for overcoming coherent Rayleigh noise is frequency dithering of the laser source to change the spectral characteristics of the source. This technique is effective to the extent that it reduces the amount of noise power that falls within the receiver bandwidth. This frequency dithering technique is described in greater detail in a paper by T. H. Wood, R. A. Linke, B. L. Kaspar, and E. C. Carr, entitled, "Observation of Coherent Rayleigh Noise in Single-Source Bi-directional Optical Fiber System," Journ. Lightwave Techn., Vol. 6, pp. 346-352 (1988). Unfortunately, this method leads to an incomplete suppression of the coherent Rayleigh noise. Since the electrical bandwidth of the receiver is tied to the bit rate, this method is not effective when the bandwidth of the communication system becomes larger than the spectral broadening that can be obtained by dithering.
Yet another technique which addresses the problem of coherent Rayleigh noise is to shift the frequency of the light at the location of the modulator. This may be accomplished by using an acousto-optic modulator, so that up- and downstream wavelengths are different, thus ideally eliminating interference altogether. This approach is described in U.S. Pat. No. 5,572,612, entitled, "Bidirectional Optical Transmission System". However, changing the frequency of light at the modulator increases the complexity and cost of the optical network unit (ONU) at the customer premises, contrary to the original intent of saving cost and complexity by replacing a laser with a modulator at the ONU. Moreover, since a typical system may utilize many ONUs for each central office laser, the cost of furnishing a special high-frequency modulator to each ONU is multiplied by the relatively higher number of ONUs that are used relative to each central office. What is needed is an improved technique for overcoming coherent Rayleigh noise in a bidirectional optical communications system.